Routing area updating in packet radio network

ABSTRACT

The invention relates to a cellular packet radio network and to a method for updating a routing area in a packet radio network. Packet radio support nodes (SGSN) are connected to a digital cellular radio network (BSS), which provides a radio interface for the support nodes for packet-switched data transmission between the support nodes and mobile stations. There is a logical link between a mobile station (MS) and a serving packet radio support node (SGSN). The packet radio network utilized logical routing areas, each of which comprises one or more cell. Each cell broadcasts information on the routing area to which it belongs. The mobile station sends a routing area update request to the packet radio network when it roams to a new cell which belongs to a different routing area than the old cell. The update request includes the identifiers of the old and new routing area. When the packet radio node detects a routing area update carried out by an unknown mobile station, it initiates the establishment of a logical link by sending a link establishment message (LLC Subm,  21, 21′ ) to the mobile station, the message including the same identifier the mobile used for itself in the routing area update request. The mobile station initializes the logical link at its own end and sends and acknowledgement to the serving packet radio support node.

This application is the national phase of international applicationPCT/FI98/00040 filed Jan. 19, 1998 which designated the U.S.

FIELD OF THE INVENTION

The invention relates to packet radio networks in general, and inparticular to supporting mobility in packet radio networks.

BACKGROUND OF THE INVENTION

Mobile communication systems have been developed because it has beennecessary to be able to reach people even when they are not close to afixed telephone terminal. As the use of various data transmissionservices in offices has increased, different data services have alsobeen introduced into mobile communication systems. Portable computersenable effective data processing wherever the user moves. Mobilecommunication networks in turn provide an effective access network toactual data networks for the user for mobile data transmission. Torealize this, data services of new kind are designed for existing andfuture mobile communication networks. Mobile data transmission issupported particularly well by digital mobile communication systems,such as the pan-European mobile communication system GSM (Global Systemfor Mobile Communication).

The general packet radio service GPRS is a new service in the GSMsystem, and is one of the objects of the standardization work of the GSMphase 2+ at ETSI (European Telecommunication Standard Institute). TheGPRS operational environment comprises one or more subnetwork serviceareas, which are interconnected by a GPRS backbone network. A subnetworkcomprises a number of packet data service nodes SN, which in thisapplication will be referred to as serving GPRS support nodes SGSN, eachof which is connected to the GSM mobile communication network (typicallyto base station systems) in such a way that it can provide a packetservice for mobile data terminals via several base stations, i.e. cells.The intermediate mobile communication network provides packet-switcheddata transmission between a support node and mobile data terminals.Different subnetworks are in turn connected to an external data network,e.g. to a public switched data network PSPDN, via GPRS gateway supportnodes GGSN. The GPRS service thus allows to provide packet datatransmission between mobile data terminals and external data networkswhen the GSM network functions as an access network. The GPRS networkarchitecture is illustrated in FIG. 1.

In the GPRS system, layered protocol structures, known as a transmissionlevel and a signalling level, have been defined for transmitting userinformation and signalling. A transmission level has a layered protocolstructure providing transmission of user information together withcontrol procedures of data transmission related to it (e.g. flowcontrol, error detection, error correction and error recovery). Asignalling level consists of protocols which are used for controllingand supporting the functions of the transmission level, such ascontrolling access to the GPRS network (Attach and Detach) andcontrolling the routing path of the established network connection inorder to support the user's mobility. FIG. 2 illustrates the signallinglevel of the GPRS system between an MS and an SGSN. The protocol layersof the transmission level are identical with those of FIG. 2 up toprotocol layer SNDCP, above which there is a protocol of the GPRSbackbone network (e.g. Internet Protocol IP) between the MS and the GGSN(instead of protocol L3MM). The protocol layers illustrated in FIG. 2are:

Layer 3 Mobility Management (L3MM): This protocol supports thefunctionality of mobility management, e.g. GPRS Aftach, GPRS Detach,security, routing update, location update, activation of a PDP context,and deactivation of a PDP context.

Subnetwork Dependent Convergence Protocol (SNDCP) supports transmissionof protocol data units (N-PDU) of a network layer between an MS and anSGSN. The SNDCP layer, for example, manages ciphering and compression ofN-PDUs.

Logical Link Control (LLC); this layer provides a very reliable logicallink. The LLC is independent of the radio interface protocols mentionedbelow.

LLC Relay: This function relays LLC protocol data units (PDU) between anMS-BSS interface (Um) and a BSS-SGSN interface (Gb).

Base Station Subsystem GPRS Protocol (BSSSGP): This layer transmitsrouting information and information related to QoS between a BSS and anSGSS.

Frame Relay, which is used over the Gb interface. A semipermanentconnection for which several subscribers' LLC PDUs are multiplexed isestablished between the SGSN and the BSS.

Radio Link Control (RLC): This layer provides a reliable linkindependent of radio solutions.

Medium Access Control (MAC): This one controls access signalling(request and grant) related to a radio channel and mapping of LLC framesonto a physical GSM channel.

With respect to the invention the most interesting protocol layers arethe LCC and L3MM. The function of the LLC layer can be described asfollows: the LLC layer functions above the RLC layer in the referencearchitecture and establishes a logical link between the MS and itsserving SGSN. With respect to the function of the LCC the most importantrequirements are a reliable management of LCC frame relay and supportfor point-to point and point-to-multipoint addressing.

The service access point (SAP) of the logical link layer is a pointwhere the LLC layer provides services for the protocols of layer 3(SNDCP layer in FIG. 2). The link of the LLC layer is identified with adata link connection identifier (DLCI), which is transmitted in theaddress field of each LLC frame. The DLCI consists of two elements:Service Access Point Identifier (SAPI) and Terminal End Point Identifier(TEI). The TEI identifies a GPRS subscriber and is usually a TemporaryLogical Link Identity TLLI. The TEI can also be another subscriberidentity, such as an international mobile subscriber identity IMSI, butusually transmission of the IMSI on the radio path is avoided.

When a user attaches to a GPRS network, a logical link is establishedbetween the MS and the SGSN. Thus it can be said that the MS has a callin progress. This logical link has a route between the MS and the SGSN,indicated with the TLLI identifier. Thus the TLLI is a temporaryidentifier, the SGSN of which allocates for a certain logical link andIMSI. The SGSN sends the TLLI to the MS in connection with theestablishment of a logical link, and it is used as an identifier inlater signalling and data transmission over this logical link.

Data transmission over a logical link is carried out as explained in thefollowing. The data to be transmitted to or from an MS is processed withan SNDCP function and transmitted to the LLC layer. The LLC layerinserts the data in the information field of LLC frames. The addressfield of a frame includes e.g. a TLLI. The LLC layer relays the data tothe RLC, which deletes unnecessary information and segments the datainto a form compatible with the MAC. The MAC layer activates radioresource processes in order to obtain a radio traffic path fortransmission. A corresponding MAC unit on the other side of the radiotraffic path receives the data and relays it upwards to the LLC layer.Finally, the data is transmitted from the LLC layer to the SNDCP, wherethe user data is restored completely and relayed to the next protocollayer.

The LLC layer controls transmission and retransmission of LLC framesover a logical link. Several state variables are related to thecontrolling at both ends of the link. In multiframe transmission suchstate variables include e.g. a transmission state variable V(S),acknowledgement state variable V(A), transmission sequence number N(S),receiving state variable V(R), and receiving sequence number N(R). TheV(S) indicates the number of the frame to be transmitted next. The V(A)indicates the number of the last frame the opposite end hasacknowledged. The V(S) shall not exceed the V(A) by more than k frames,i.e. the size of the transmission window is k. The V(R) indicates thenumber of the next frame that is expected to be received. The statevariables are reset, i.e. set to value 0 when a logical link is beingestablished. This is performed by using the following messages of theLAPG (Link Access Procedure on the “G” channel) protocol: SABM (SetAsynchronous Balanced Mode) and UA (Unnumbered Acknowledgement) or SAUM(Set Asynchronous Unbalanced Mode).

Three different MM states of the MS are typical of the mobilitymanagement (MM) of a GPRS subscriber: idle state, standby state andready state. Each state represents a certain functionality andinformation level, which has been allocated to the MS and SGSN.Information sets related to these states, called MM contexts, are storedin the SGSN and MS. The context of the SGSN contains subscriber data,such as the subscriber's IMSI, TLLI and location and routinginformation, etc.

In the idle state the MS cannot be reached from the GPRS network, and nodynamic information on the current state or location of the MS, i.e. onthe MM context, is maintained in the network. Neither does the MSreceive nor transmit data packets, in consequence of which no logicallink has been established between the SGSN and the MS. If the MS is adual-mode terminal, i.e. it can function both in the GPRS network and inthe GSM network, it can be in the GSM network when functioning in theGPRS idle state. The MS can switch from the idle state to the readystate by attaching to the GPRS network, and from the standby or readystate to the idle state by detaching from the GPRS network.

In the standby and ready states the MS is attached to the GPRS network.In the GPRS network, a dynamic MM context has been created for the MS,and a logical link LLC (Logical Link Control) established between the MSand the SGSN in a protocol layer. The ready state is the actual datatransmission state, in which the MS can transmit and receive user data.The MS switches from the standby state to the ready state either whenthe GPRS network pages the MS or when the MS initiates data transmissionor signalling. The MS may remain in the ready state (for a period setwith a timer) even when no user data is transmitted nor signallingperformed.

In the standby and ready states the MS also has one or more PDP contexts(Packet Data Protocol), which are stored in the serving SGSN inconnection with the MM context. The PDP context defines different datatransmission parameters, such as the PDP type (e.g. X.25 or IP), PDPaddress (e.g. X.121 address), quality of service QoS and NSAPI. The MSactivates the PDU context with a specific message, Activate PDP ContextRequest, in which it gives information on the TLLI, PDP type, PDPaddress, required QoS and NSAPI. When the MS roams to the area of a newSGSN, the new SGSN requests MM and PDP contexts from the old SGSN.

For mobility management, logical routing areas have been defined to theGPRS network. A routing area RA is an area defined by an operator,comprising one or more cells. Usually, one SGSN serves several routingareas. A routing area is used for determining the location of the MS inthe standby state. If the location of the MS is not known in terms of aspecific cell, signalling is started with a GPRS page within one routingarea RA.

The MS performs a routing area update procedure in order to supportmobility of a packet-switched logical link. In the READY state the MSinitiates the procedure when a new cell is selected, the routing areachanges or the update timer of a cyclic routing area expires. The radionetwork (PLMN) is arranged to transmit a sufficient amount of systeminformation to the MS so that it can detect when it enters a new cell ora new routing area RA and to determine when it is to carry out cyclicrouting area updates. The MS detects that it has entered a new cell bycomparing cyclically the cell identity (Cell ID) which is stored in itsMM context with the cell identity which is received from the network.Correspondingly, the MS detects that it has entered a new routing areaRA by comparing the routing area identifier stored in its MM contextwith the routing area identifier received from the network. When the MSselects a new cell, it stores the cell identity and routing area in itsMM context.

All the procedures described above (e.g. attach, detach, routing areaupdate and activation/deactivation of the PDP context) for creating andupdating MM and PDP contexts and establishing a logical link areprocedures activated by the MS. In connection with a routing area updatethe MS, however, carries out an update to the new routing area withoutbeing able to conclude on the basis of the routing area informationbroadcast by cells whether the SGSN serving the new cell is the same asthe SGSN that served the old cell. On the basis of the old routing areainformation transmitted by the MS in an update message the new SGSNdetects that a routing area update is in progress between SGSNs andactivates necessary interrogations to the old SGSN in order to createnew MM and PDP contexts for the MS to the new SGSN. Since the SGSN haschanged, the logical link should be re-established between the MS andthe new SGSN. The problem is, however, that the MS does not know thatthe SGSN has changed. Instead, the MS may transmit data to the new SGSN,which is not able to unpack the data before MM and PDP contexts havebeen created on the basis of the inquiry made to the old SGSN.Furthermore, even though the contexts were already created in the newSGSN, state variables at the ends of the logical link do not match anddata transmission fails, at least temporarily. If the retrieval of PDPcontexts from the old SGSN fails, data transmission is prevented, sincethe new SGSN does not serve according to the PDP context activatedearlier by the MS. The MS, however, has no information on such asituation, and it cannot start reactivation of the PDP context.

DISCLOSURE OF THE INVENTION

An object of the invention is to minimise and eliminate the problems anddisadvantages resulting from the routing area update.

The invention relates to a method for updating a routing area in apacket radio network, the method comprising the steps of

establishing a logical link between a mobile station and a first radiosupport node via a radio cell,

the mobile station selecting a second radio cell on roaming, the routingarea identifier broadcast by the second cell being different from therouting area identifier of the first radio cell,

sending a routing area update request to the second packet radio supportnode serving the second cell from the mobile station,

detecting in the second packet radio support node that the mobilestation has roamed from a routing area served by a different packetradio support node,

requesting subscriber data related to the mobile station from the firstpacket radio support node,

sending an acknowledgement message on the routing area update from thesecond packet radio support node to the mobile station. The method ischaracterized by

initiating a signalling procedure for initializing a logical linkbetween the mobile station and the second packet radio support node inthe second packet radio support node in response to detecting that themobile station has roamed from a routing area served by a differentpacket radio support node.

The basic idea of the invention is that the serving packet radio supportnode, which detects a routing area update performed by an unknown mobilestation, initiates the establishment of a logical link by sending a linkestablishment message to the mobile station, the message containing thesame identifier the mobile station used for itself in the routing areaupdate request. The mobile station initializes the logical link at itsown end and sends an acknowledgement to the serving packet radio supportnode. The initialization of the logical link performed by the mobilestation may comprise resetting the state variables of the logical linkand changing the new identifier possibly sent by the support node in themobile station. Default values, for example, can be used for otherpossible link parameters.

The establishment message of a logical link may be any message or asequence of messages indicating the establishment or re-initializationof a logical link. It is preferably possible to use messages similar tothe ones the mobile station employs for initiating the establishment ofa logical link on attaching to the network for the first time. In apreferred embodiment of the invention the serving packet radio node SGSNof the GPRS system sends a SABM command (Set Asynchronous Balanced Mode)of the LAPG protocol (Link Access Procedure on the “G” channel) to themobile station, and the mobile station acknowledges it by sending an UAresponse (Unnumbered acknowledgement). Another preferred solution is touse the negotiation procedure of LLC link parameters of the GPRS systemin such a way that the serving support node sends a XID command to themobile station, the command including a parameter message. The mobilestation sends a XID reply message including a list of the parameters itsupports. When resetting of the state variables according to theinvention (or another kind of initialization of the logical link) isadded to this in the mobile station in response to the XID command,desirable link parameters can be provided with very little signalling.

As was stated above, in some exceptional situations the packet dataprotocol context (PDP) cannot be retrieved for the new support node fromthe old support node. This may result e.g. from the fact that the newsupport node does not recognize the old support node or cannot contactit, or the old support node has lost the subscriber data. In that case,in one preferred embodiment of the invention, the new support node sendsinformation to the mobile station that it should initiate the activationprocedure of a PDP context (contexts). This information can be relayedas included in the information on the change of the support node or asseparate information. Without this procedure the new support node cannotfunction in the way defined earlier by the mobile station, and datatransmission is prevented at least temporarily without the mobilestation detecting this.

According to one embodiment of the invention, the new support nodeinitiates the establishment of a logical link immediately after havingdetected that the support node has changed. Thus the establishmentsignalling of a logical link can be performed at the same time as thenew support node searches for subscriber data from the old support nodeor the subscriber data base. An advantage resulting from this is thatthe mobile station does not try to transmit more data packets since itthinks that it still has a link to the old support node. Furthermore,the update of the logical link can be initiated as early as possible,which speeds up the initiation of data transmission after subscriberdata have been received in the support node. If the new support nodedoes not accept the link to the mobile station after the subscriber datahave been checked, the support node disconnects the established logicallink by sending a disconnect message to the mobile station. In the GPRSsystem a message suitable for this purpose is, for example, the DISCcommand (Disconnect) of the LAPG protocol.

The invention provides an effective way for re-establishing a logicallink between support nodes in connection with a routing area update. Theinvention allows to completely avoid the update of the logical link andinterruptions to data transmission in connection with an intra-supportnode routing area update. The last-mentioned problem would arise, forexample, in such a solution in which the mobile station would alwaysinitiate re-establishment of a logical link in connection with a routingarea update as a precaution.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail bymeans of preferred embodiments with reference to the accompanyingdrawings, in which

FIG. 1 illustrates GPRS network architecture,

FIG. 2 illustrates protocol layers of the signalling level between an MSand an SGSN,

FIG. 3 is a signalling diagram illustrating a routing area updateaccording to the invention.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention can be applied to packet radio systems of variouskind. The invention can be especially preferably used for providing ageneral packet radio service GPRS in the pan-European digital mobilecommunication system GSM (Global System for Mobile Communication) or incorresponding mobile communication systems, such as DCS1800 and PCS(Personal Communication System). In the following, the preferredembodiments of the invention will be described by means of a GPRS packetradio network formed by the GPRS service and the GSM system withoutlimiting the invention to this particular packet radio system.

FIG. 1 illustrates a GPRS packet radio network implemented in the GSMsystem.

The basic structure of the GSM system comprises two elements: a basestation system BSS and a network subsystem NSS. The BSS and mobilestations MS communicate over radio links. In the base station system BSSeach cell is served by a base station BTS. A number of base stations areconnected to a base station controller BSC, which controls the radiofrequencies and channels used by the BTS. Base station controllers BSCare connected to a mobile services switching centre MSC. As regards amore detailed description of the GSM system, reference is made to theETSI/GSM recommendations and The GSM System for Mobile Communications,M. Mouly and M. Pautet, Palaiseau, France, 1992, ISBN:2-957190-07-7.

In FIG. 1 the GPRS system connected to the GSM network comprises oneGPRS network, which in turn comprises two serving GPRS support nodes(SGSN) and one GPRS gateway support node (GGSN). The different supportnodes SGSN and GGSN are interconnected by an intra-operator backbonenetwork. It is important to realize that in the GPRS network there maybe any number of support nodes and gateway support nodes.

The serving GPRS support node SGSN is a node which serves the mobilestation MS. Each support node SGSN controls a packet data service withinthe area of one or more cells in a cellular packet radio network, andtherefore, each support node SGSN is connected (Gb interface) to acertain local element of the GSM system. This connection is typicallyestablished to the base station system BSS, i.e. to base stationcontrollers BSC or to a base station BTS. The mobile station MS locatedin a cell communicates with a base station BTS over a radio interfaceand further with the support node SGSN to the service area of which thecell belongs through the mobile communication network. In principle, themobile communication network between the support node SGSN and themobile station MS only relays packets between these two. To realize thisthe mobile communication network provides packet-switched transmissionof data packets between the mobile station MS and the serving supportnode SGSN. It has to be noted that the mobile communication network onlyprovides a physical connection between the mobile station MS and thesupport node SGSN, and thus its exact function and structure is notsignificant with respect to the invention. The SGSN is also providedwith a signalling interface Gs to the visitor location register VLR ofthe mobile communication network and/or to the mobile services switchingcentre, e.g. signalling connection SS7. The SGSN may transmit locationinformation to the MSC/VLR and/or receive requests for paging a GPRSsubscriber from the MSC/VLR.

When the MS attaches to the GPRS network, i.e. in a GPRS attachprocedure, the SGSN creates a mobility management context (MM context)containing information related to the mobility and security of the MS,for example. In connection with a PDP activation procedure the SGSNcreates a PDP context (packet data protocol) which is used for routingpurposes within the GPRS network with the GGSN which the GPRS subscriberuses.

The GPRS gateway support node GGSN connects an operator's GPRS networkto other operators' GPRS systems and to data networks 11-12, such as aninter-operator backbone network, IP network (Internet) or X.25 network.The GGSN includes GPRS subscribers' PDP addresses and routinginformation, i.e. SGSN addresses. Routing information is used fortunneling protocol data units PDU from data network 11 to the currentswitching point of the MS, i.e. to the serving SGSN. Functionalities ofthe SGSN and GGSN can be integrated into the same physical node.

The home location register HLR of the GSM network contains GPRSsubscriber data and routing information and maps the subscriber's IMSIinto one or more pairs of the PDP type and PDP address. The HLR alsomaps each PDP type and PDP address pair into one or more GGSNs. The SGSNhas a Gr interface to the HLR (a direct signalling connection or via aninternal backbone network 13). The HLR of a roaming MS may be in adifferent mobile communication network than the serving SGSN.

An intra-operator backbone network 13, which interconnects an operator'sSGSN and GGSN equipment can be implemented, for example, by means of alocal network, such as an IP network. It should be noted that anoperator's GPRS network can also be implemented without theintra-operator backbone network, e.g. by providing all features in onecomputer.

An inter-operator backbone network is a network via which differentoperators' gateway support nodes GGSN can communicate with one another.

In the following, the routing area update in the GPRS network will bedescribed in greater detail. The MS carries out a routing area updateprocedure in order to support mobility of a packet-switched logicallink. The MS initiates the procedure when a new cell is selected and therouting area RA changes, or when the cyclic update timer of a routingarea expires. The radio network (PLMN) is arranged to transmit asufficient amount of system information to the MS so that it can detectwhen it enters a new cell or routing area RA and to determine when it isto carry out cyclic routing area updates. The MS detects that it hasentered a new cell by comparing cyclically the cell identity (Cell ID)which is stored in its MM context with the cell identity which isreceived from the network. Correspondingly, the MS detects that it hasentered a new routing area RA by comparing the routing area identifierstored in its MM context with the routing area identifier received fromthe network.

When the MS detects a new cell or a new routing area RA, this means thatone of the three possible cases is in question: 1) a cell update isneeded; 2) a routing area update is needed; and 3) a combined update ofa routing area and location area is needed. In all these three cases theMS selects a new cell locally and stores the cell identity in its MMcontext.

Cell Update Procedure

A cell update is performed when the MS enters a new cell within thecurrent routing area RA and is in the READY state. If the RA haschanged, a routing area update is carried out instead of the cellupdate.

The cell update procedure is carried out as an implicit procedure at theLLC level, which means that normal LLC information and control framesare used for sending information on crossover to the SGSN. Intransmission toward the SGSN, the cell identity is added to all LLCframes in the base station system of the network. The SGSN registers thecrossover of the MS, and any further traffic toward the MS is routed viaa new cell. In a simple cell update the SGSN does not change, andproblems overcome by the invention will not arise.

Update Procedure of Routing Area

A routing area is updated when an MS attached to the GPRS networkdetects that it has entered a new routing area RA, or when the cyclicalRA update timer has expired. When the SGSN detects that it is alsocontrolling the old routing area, it finds out that an intra-SGSNrouting update is in question. In this case the SGSN has the necessaryinformation on the MS, and it does not need to inform the GGSNs, HLR orMSC/VLR of the new location of the MS. The cyclical RA update is alwaysan intra-SGSN routing area update. In the intra-SGSN routing updateprocedure the SGSN will not change either, and no problems overcome bythe invention will arise.

Inter-SGSN Routing Area Update

In the routing area update procedure between two SGSNs (inter-SGSN) theserving SGSN changes and, according to the invention, the MS should beinformed of the change so that the MS can initiate a local procedure ora network procedure for updating a logical link. In the following, theinter-SGSN routing area update according to a first embodiment of theinvention will be described with reference to FIG. 3. In the followingdescription the reference numbers refer to messages or events shown inFIG. 3.

1. The MS sends a routing area update request to the SGSN. This messageincludes the temporary logical link identity TLLI, cell identity of thenew cell Cellid, routing area identifier of the old routing area RAid,and routing area identifier of the new routing area RAid. if load is tobe decreased in the radio interface, the cell identity Cellid is notadded until in the base station system BSS.

2. The new SGSN detects that the old routing area belongs to anotherSGSN, which will be referred to as an old SGSN in this context. As aresult, the new SGSN requests MM and PDP contexts for the MS in questionfrom the old SGSN. All contexts can be requested at the same time, orthe MM context and each PDP context can be requested in differentmessages. The request (requests) includes at least the routing areaidentifier RAId of the old routing area and the TLLI. The old SGSN sendsin response an MM context, PDP contexts and possibly authorizationparameter triplets. If the MS is not recognized in the old SGSN, the oldSGSN replies with an appropriate error message. The old SGSN stores thenew SGSN address until the old MM context has been deleted so that datapackets can be relayed from the old SGSN to the new SGSN.

3. The new SGSN sends a message “Modify PDP Context Request” includinge.g. a new SGSN address to the GGSNs concerned. The GGSNs update theirPDP context fields and send in response a message “Modify PDP ContextResponse”.

4. The SGSN informs the HLR of the change of the SGSN by sending it amessage “Update Location” including a new SGSN address and IMSI.

5. The HLR deletes the MM context from the old SGSN by sending it amessage “Cancel Location” including an IMSI. The old SGSN deletes the MMand PDP contexts and acknowledges this by sending a message “CancelLocation Ack”.

6. The HLR sends a message “Insert Subscriber Data” including an IMSIand GPRS subscriber data to the new SGSN. The SGSN acknowledges this bysending a message “Insert Subscriber Data Ack”.

7. The HLR acknowledges the location update by sending a message “UpdateLocation Ack” to the SGSN.

8. If the subscriber is also a GSM subscriber (IMSI-Attached), theassociation between the SGSN and the VLR has to be updated. The VLRaddress is deduced from the RA information. The new SGSN transmitsmessages “Location Updating Request” including e.g. an SGSN address andIMSI to the VLR. The VLR stores the SGSN address and acknowledges bysending a message “Location Updating Accept”.

9. The new SGSN confirms the presence of the MS in the new routing areaRA. If there are no restrictions for registration of the MS for the newRA, the SGSN creates MM and PDP contexts for the MS. A logical link willbe established between the new SGSN and the MS. The new SGSN replies tothe MS with a message “Routing Area Update Accept” including e.g. a newTLLI. This message tells to the MS that the network has succeeded incarrying out the update.

10. The MS acknowledges the new TLLI with a message “Routing Area UpdateComplete”.

21. In a preferred embodiment of the invention the new SGSN whichdetects that the. old location area identifier belongs to another SGSNimmediately initiates the establishment of a logical link by sending aSABM (Set Asynchronous Balanced Mode) command of the LAPG protocol (LinkAccess Procedure on the “G” channel) to the mobile station. This commandincludes the same TLLI the mobile station used in the Routing AreaUpdate Request message. On receiving the SABM command the mobile stationresets the state variables ((VS), V(A) and V(R) to value 0), countersand timers. In the invention the reception of the SABM commandpreferably also comprises setting LLC link parameters to their defaultvalues in the MS. Such LLC link parameters include the maximum delay(T200) of the acknowledgement response of a data frame, maximum number(N200) of retransmissions of a frame, maximum number (N201) of octets inthe information field of a frame, and maximum number of sentunacknowledged frames, i.e. the size of the acknowledgement window is K.Resetting of state variables according to the invention also comprises atimer measuring the delay T200 and resetting of a retransmission counterN200. In general, it can be said that the update of a logical linkcomprises all necessary initializations in different protocol layers inthe MS. The MS replies by sending a UA response (UnnumberedAcknowledgement) to the SGSN. The SGSN performs the initialization ofthe state variables, link parameters, counters and timers at its ownend, and thus the logical link has been established. The MS alsopreferably totally discontinues data transmission to the SGSN until ithas received a Routing Area Accept message from the SGSN in step 9.

In another embodiment of the invention the new MS, which detects thatthe old location area identifier belongs to another SGSN, immediatelystarts the establishment of a logical link by initiating a procedure ofthe LLC layer, which the MS and SGSN employ for negotiating said LLClink parameters. For this purpose the MS sends a XID command accordingto the LAPG protocol to the SGSN, the command including a parametermessage. The parameter message contains the values requested by the MSfor LLC link parameters T200, N200, N201 and K. The SGSN sends a XIDresponse containing a list of the parameter values the SGSN supports.The MS and SGSN set these values to the parameters.

Naturally, the SGSN may also use another suitable signalling sequencefor initiating the establishment of a logical link in the LLC layer orin another protocol layer.

21′. The establishment of a logical link described in item 21 may bealternatively performed at any point, e.g. after the Routing Area Acceptmessage has been sent, as illustrated with LLC signalling 21′ in FIG. 3.However, this delays the initiation of data transmission after theupdate procedure.

11. In another embodiment of the invention the SGSN includes informationthat the MS should initiate the activation of the PDP context (contexts)in the Routing Area Accept message (e.g. in the Cause field). The SGSNadds this information at least if the search for a PDP context(contexts) from the old SGSN has failed. When the MS receives theinformation “activate PDP contexts”, it performs the update of a logicallink described above and initiates the activation of a PDP context(contexts) by sending a message (messages) “Activate PDP ContextRequest” to the SGSN.

Standardization of the GPRS system has not been completed yet. Thepresent state of the GPRS system is described in recommendations GSM03.60 version 0.20.0 and GSM 04.64 version 0.0.1(DRAFT) of the EuropeanTelecommunications Standards Institute (ETSI), which are incorporatedherein by reference.

The description only illustrates preferred embodiments of the invention.The invention is not, however, limited to these examples, but it mayvary within the scope and spirit of the appended claims.

What is claimed is:
 1. A method for updating a routing area in a packetradio network, the method comprising: establishing a logical linkbetween a mobile station and a first packet radio support node via afirst radio cell, the mobile station selecting a second radio cell onroaming, the routing area identifier broadcast by the second cell beingdifferent from the routing area identifier of the first radio cell, themobile station sending a routing area update request to the secondpacket radio support node serving the second cell, detecting, in thesecond packet radio support node, that the mobile station has roamedfrom a routing area served by a different packet radio support node,requesting subscriber data related to the mobile station from the firstpacket radio support node, sending an acknowledgement message of therouting area update from the second packet radio support node to themobile station, initiating a signaling procedure for initializing alogical link between the mobile station and the second packet radiosupport node in the second packet radio support node in response todetecting that the mobile station has roamed from a routing area servedby a different packet radio support node.
 2. A method as claimed inclaim 1, wherein said local initialization of a logical link comprisesresetting of the state variables of the logical link in the mobilestation.
 3. A method as claimed in claim 1 or 2, wherein said localinitialization of a logical link comprises setting the link parametersof the logical link to their default values in the mobile station.
 4. Amethod as claimed in any one of claims 1 to 2, wherein said networkprocedure for initializing a logical link comprises the steps of sendinga first message of the logical link protocol from the second packetradio support node to the mobile station, resetting the state variablesof the logical link in the mobile station in response to said firstmessage, sending a response message from the mobile station to thesecond packet radio support node, resetting the state variables of thelogical link in the second packet radio support node.
 5. A method asclaimed in claim 4, wherein said packet radio network is a GPRS network,and said first message is the Set Asynchronous Balanced Mode command ofthe LAPG protocol, and said response message is the UnnumberedAcknowledgement response of the LAPG protocol.
 6. A method as claimed inany one of claims 1 to 2, wherein said signaling procedure forinitializing a logical link comprises the steps of performingnegotiation of link parameters according to the logical link protocolbetween the mobile station and the second packet radio support node,setting the link parameters to the negotiated values in the mobilestation and second support node, resetting the state variables of thelogical link in the mobile station.
 7. A method as claimed in claim 6,wherein the packet radio network is a GPRS network, and said negotiationof link parameters comprises the steps of sending a XID command of theLAPG protocol from the second packet radio support node to the mobilestation, the command including a list of the link parameter valuessupported by the second packet radio support node, sending a XIDresponse of the LAPG protocol from the mobile station to the secondpacket radio support node, the response including a list of the linkparameter values requested by the mobile station, setting the linkparameter values to the values given in the XID command in the mobilestation and second packet radio support node, resetting the statevariables of the logical link in the mobile station.
 8. A method asclaimed in claim 1 or 2, wherein the signaling procedure forinitializing a logical link is performed before, simultaneously with, orafter requesting subscriber data from the first packet radio supportnode.
 9. A method for updating a routing area in a packet radio network,the method comprising the steps of establishing a logical link between amobile station and a first radio support node via a radio cell, themobile station selecting a second radio cell on roaming, the routingarea identifier broadcast by the second cell being different from therouting area identifier of the first radio cell, sending a routing areaupdate request to the second packet radio support node serving thesecond cell from the mobile station, detecting in the second packetradio support node that the mobile station has roamed from a routingarea served by a different packet radio support node, requestingsubscriber data related to the mobile station from the first packetradio support node, sending an acknowledgement message on the routingarea update from the second packet radio support node to the mobilestation, initiating a signaling procedure for initializing a logicallink between the mobile station and the second packet radio support nodein the second packet radio support node in response to detecting thatthe mobile station has roamed from a routing area served by a differentpacket radio support node, failing to retrieve packet data protocolparameters which the mobile station has activated in the first packetradio support node from the first packet radio support node for thesecond packet radio support node, sending information from the secondpacket radio support node to the mobile station that it shouldreactivate the packet data protocol parameters, sending a message foractivating the packet data protocol parameters from the mobile stationto the second packet radio support node.
 10. A method as claimed inclaim 9, wherein the packet radio network is a GPRS network, and saidactivation message is Activate PDP Context Request.
 11. A method asclaimed in claim 9, wherein the packet radio network is a GPRS network,and said information on the need for activating the packet data protocolparameters is sent in a message of the mobility management protocol,e.g. in the MM message Routing Area Update Accept.
 12. A cellular packetradio network, comprising mobile stations, packet radio support nodes,which are connected to a digital cellular radio network providing aradio interface for the support nodes for packet-switched datatransmission between the support nodes and mobile stations, there beinga logical link between the mobile station and the serving packet radiosupport node, logical routing areas, each of which comprises one or morecells in the radio network, each cell being arranged to broadcastinformation on the routing area to which it belongs, the mobile stationsbeing arranged to send a routing area update request to the packet radionetwork when they roam to a new cell which belongs to a differentrouting area than the old cell, said update request includingidentifiers of the old and new routing area, the packet radio supportnode being arranged to request the mobile subscriber's subscriber datafrom the old packet radio support node upon detecting that the oldrouting area belongs to a different packet radio support node, thepacket radio support node being arranged to acknowledge the routing areaupdate to the mobile station when the node has received the subscriberdata, the packet radio support node being arranged to initiate asignaling procedure for initializing a logical link between the mobilestation and the second packet radio support node in response todetecting that the mobile station has roamed from a routing area servedby a different packet radio support node.
 13. A packet radio network asclaimed in claim 12, wherein the packet radio support node is arrangedto initiate said signaling procedure before or at the same time as itretrieves the subscriber data from the old packet radio support node.14. A packet radio network as claimed in claim 12 or 13, wherein theinitialization of a logical link comprises resetting the state variablesof the logical link and setting the link parameters of the logical linkto their default values in the mobile station.
 15. A packet radionetwork as claimed in any one of claims 12 to 13, wherein said signalingprocedure for initializing a logical link comprises a message of thefirst logical link protocol sent from the radio support node, themessage causing resetting of the state variables of the logical link inthe mobile station, and a response message sent from the mobile stationto the second packet radio support node.
 16. A packet radio network asclaimed in claim 15, wherein said packet radio network is a GPRSnetwork, and said first message is the Set Asynchronous Balanced Modecommand of the LAPG protocol and said response message is the UnnumberedAcknowledgement response of the LAPG protocol.
 17. A packet radionetwork as claimed in any one of claims 12 to 13, wherein said signalingprocedure for initializing a logical link comprises negotiation of linkparameters according to a logical link protocol between the secondpacket radio support node and the mobile station, the negotiationcausing setting of the link parameters to the negotiated values in themobile station and second support node, and resetting of the statevariables of the logical link in the mobile station.
 18. A packet radionetwork as claimed in claim 17, wherein the packet radio network is aGPRS network, and said negotiation of the link parameters comprises aXID command of the LAPG protocol sent from the second packet radiosupport node to the mobile station, the command including a list of thelink parameter values supported by the second packet radio support node,and a XID response of the LAPG protocol sent from the mobile station tothe second packet radio support node, the response including a list ofthe link parameter values requested by the mobile station, and thenegotiation causing setting of the link parameters to the values givenin the XID response in the mobile station and second packet radiosupport node, and resetting of the state variables of the logical linkin the mobile station.
 19. A packet radio network as claimed in claim12, wherein the acknowledgement message on the routing area updateincludes said information on the change of the packet radio supportnode.
 20. A packet radio network comprising mobile stations, packetradio support nodes, which are connected to a digital cellular radionetwork providing a radio interface for the support nodes forpacket-switched data transmission between the support nodes and mobilestations, there being a logical link between the mobile station and theserving packet radio support node, logical routing areas, each of whichcomprises one or more cells in the radio network, each cell beingarranged to broadcast information on the routing area to which itbelongs, the mobile stations being arranged to send a routing areaupdate request to the packet radio network when they roam to a new cellwhich belongs to a different routing area than the old cell, said updaterequest including identifiers of the old and new routing area, thepacket radio support node being arranged to request the mobilesubscriber's subscriber data from the old packet radio support node upondetecting that the old routing area belongs to a different packet radiosupport node, the packet radio support node being arranged toacknowledge the routing area update to the mobile station when the nodehas received the subscriber data, the packet radio support node beingarranged to initiate a signaling procedure for initializing a logicallink between the mobile station and the second packet radio support nodein response to detecting that the mobile station has roamed from arouting area served by a different packet radio support node, saidsubscriber data include the packet data protocol parameters the mobilestation has activated in the old packet radio support node, the newpacket radio support node is arranged to send information that it shouldreactivate the packet data protocol parameters in response to theunsuccessful search for the subscriber data from the old packet radiosupport node, the mobile station is arranged to send a message foractivating the packet data protocol parameters to the packet radiosupport node in response to said activation information.